Well, I guess it's up to the situation. Obviously we all have different situations in our dojos.
If you go to an advanced class and get paired with a total beginner once or twice, it's not a big deal. But if you spend most of the advanced class trying to teach your partners how to do a more simplified version of what the teacher initially was teaching, just because your partners are in the wrong class, then it gets frustrating.
In our dojo, we have daily training, and separate classes for beginners and advanced. However, we recently started to see half of the students in the advanced classes cannot do propre ukemi yet. I think this not only spoils the fun for the advanced students but also can be potentially dangerous for the beginners (white belts).